1. Field of the Invention
The present invention relates to a paper feeder which stores a stack of recording materials, and feeds the recording materials sheet by sheet from the uppermost recording material to the downstream side, a recording apparatus for recording an image on a recording material, and a method of detecting a position of the terminal edge of a recording material.
2. Description of the Related Art
A printer is known as one form of the recording apparatus. Some of the printers are each equipped with a paper feeder for feeding the recording materials as printing sheets sheet by sheet from the uppermost recording material to a downstream side. The paper feeder includes a paper feed roller rotatably driven, and a hopper. The hopper is formed with a plate member long in the width direction of the printing sheet, and includes a fulcrum which is slanted when viewed from the side a sheet transport path of the printing sheet and is located in an upper part. When turned, it angularly moves toward the paper feed roller, and is pressed against the paper feed roller, or angularly moves apart from the paper feed roller. When printing sheets stacked in the hopper are moved upward by the hopper, the printing sheets are fed sheet by sheet from the uppermost printing sheet of the stack.
The hopper is urged by an urging device to turn in such a direction in which it is pressed against the paper feed roller. As a result, the stacked printing sheet is abutted on the paper feed roller. The hopper is provided with a release device, and is turned by the hopper release device in such a direction that it moves apart from the paper feed roller and its state is retained. The hopper is angularly moved between a sheet feeding position at which the uppermost printing sheet is abutted against the paper feed roller (paper feeding state) and a stand-by position at which it is most apart from the paper feed roller (release state). The sheet feeding position varies depending on the number of printing sheets as set (stacked).
When the hopper is moved from the stand-by position to the sheet feeding position, it is energetically turned to the paper feed roller or in a pressing-contact direction by the urging force of the urging device. With the turn, the printing sheet hits the paper feed roller, and a great sound (hitting sound) is generated from component parts around the hopper and the roller.
An angle (swing angle) developed when the hopper is moved from the stand-by position to the sheet feeding position, somewhat varies depending on the stack amount of the printing sheets as mentioned above. As the stack amount of the printing sheets is larger, the swing angle is smaller, and while the stack amount is smaller, the swing angle becomes larger. Accordingly, when the stack amount of the printing sheets is small where the swing angle is large, much time is taken for the sheet feeding operation, viz., high speed repeating sheet feeding operation cannot be performed.
An ink jet printer prints an image on a printing sheet in a manner that an operation of ejecting ink to the printing sheet while the recording head is reciprocated in the main scan direction and an operation of moving the printing sheet in the sub-scan direction are alternately repeated. The ink jet printer generally uses a sheet detector for detecting the printing sheet for the purpose of detecting a leading edge and a terminal edge of the printing sheet. Various types of sheet detectors for detecting the printing sheets are present. One of those known detectors is constructed such that when the printing sheet is fed, a rotatable lever part is projected and the printing sheet presses forward and turns the lever part to thereby detect the printing sheet.
There is known a sheet detector in which a mechanism having some movable part, not the rotatable lever part, engages with the printing sheet to thereby detect the printing sheet. This type of detector may take various forms. Any of those detectors detects the leading position of the printing sheet when the movable part of the sheet detector starts to engage with the recording sheet, and detects the terminal edge of the printing sheet when its engagement with the recording sheet ends.
The sheet detector of the type which detects the printing sheet through the engagement of the mechanism having the movable part engages with the printing sheet, has the following defect. At the instant that the terminal edge of the printing sheet leaves the movable part following the ending of the engagement of the movable part with the printing sheet, the sheet detector cannot recognize the leaving of the terminal edge. Exactly, a slight time elapses from the instant that the terminal edge of the printing sheet leaves the movable part till the movable part starts to move under its weight or urging force of the urging device as it returns to a fixed position and reaches a position at which the movable part can recognize the fact that the printing sheet does not engage with the movable part. That is, a slight time lag occurs.
If the sheet detector of the non-contact type, such as an optical sensor, is used, the time lag is almost negligible. On the other hand, in the case of the sheet detector which detects the printing sheet through the engagement of the movable part with the printing sheet, when it detects the terminal edge of the printing sheet, a fixed time lag occurs. The position of the terminal edge of the printing sheet, when detected, is shifted from its correct position by a distance that the printing sheet is transported during the delay time.
However, the detection offset of the terminal edge position of the printing sheet, which is caused by the delay time, is almost negligible since the transporting speed is relatively slow. It little affects the print quality.
In recent ink jet recording apparatuses, there is a tendency that the transporting of the printing sheet is performed at high speed in order to reduce the recording execution time. Accordingly, the detection offset of the terminal edge position of the printing sheet is not negligible. As a result, the following problems will be created: the blank part at the terminal edge of the printing sheet becomes narrow or the recording operation is continued beyond the terminal edge of the sheet.
The delay time may be reduced by narrowing the movable range of the movable part. If the movable range is too narrowed, the movable part will erroneously be moved responsive to a slight variation of the state of the printing sheet engaging the movable part, vibration and the like. In this respect, there is a limit in reducing the delay time by the narrowing the movable range of the movable part.
When the movable part is urged to a fixed position by an urging device having a strong urging force in order to increase the moving speed at which the movable part returns to the fixed position from the instant that the printing sheet leaves the movable part, the urging force is too strong. As a result, in the case of the printing sheet having a high rigidity, there is a danger that the leading edge of the printing sheet cannot move the movable part. For this reason, there is a limit in increasing the moving speed at which the movable part returns to the fixed position by urging the movable part to the fixed position by a strong urging force.
A possible solution to this problem is to uniquely correct the terminal edge position of the printing sheet based on the transporting speed of the printing sheet, which is set at the time of the recording control. However, the transporting speed of the printing sheet is not always constant since the transporting speed is repeatedly accelerated and decelerated by intermittently stopping the transporting of the printing sheet. For this reason, the terminal edge position of the printing sheet, which is uniquely computed from the transporting speed of the printing sheet that is set at the time of recording control, will be poor in precision.